AddPrefab.cs

using UnityEditor;
using UnityEngine;
 
 class AddPrefab
 {
    [MenuItem("KevinIsAwesome/AddPrefabToSelected")]
    static void AddPrefabToSelected()
    {
       Object prefab = AssetDatabase.LoadAssetAtPath("Assets/GameObject.prefab", typeof(GameObject));
       
       foreach (GameObject obj in Selection.activeGameObject) {
           GameObject go = GameObject.Instantiate(prefab, obj);
       }    
    }
}